JPH0261498B2 - - Google Patents

Abstract

PCT No. PCT/AU81/00171 Sec. 371 Date Jun. 17, 1982 Sec. 102(e) Date Jun. 17, 1982 PCT Filed Nov. 26, 1981 PCT Pub. No. WO82/01882 PCT Pub. Date Jun. 10, 1982.Process of radiation graft polymerizing a fluorinated unsaturated carboxylic acid monomer to a fluoropolymeric permselective membrane in order to improve its performance when used in an electrolysis cell.

Description

Claim 1: A method for treating a fluoropolymer permselective membrane to improve the membrane's resistance to reverse migration of hydroxide ions when the permselective membrane is used in a chlor-alkali electrolyzer, comprising: irradiating the membrane with high-energy radiation to generate free radical sites within the membrane, and then treating the membrane with a monomeric material comprising a fluorinated carboxylic acid or a derivative thereof. wherein the monomer material is graft copolymerized with the fluoropolymer to form a copolymer component within the membrane.

2. The method of claim 1, wherein the fluoropolymer membrane contains cation exchange groups selected from sulfonates, carboxylates and phosphonates.

3 The monomer substance has the following formula: CF ₂ =CF-X-COOR {wherein X is (CF ₂ ) _a or O-(CF ₂ -CF ₂ -O) _b
−(CF ₂ ) _c (wherein a is 0 or an integer from 1 to 6, and b
is 0 or an integer from 1 to 4 and c is an integer from 1 to 6), and R is hydrogen or a C ₁ -C ₆ alkyl group} 3. The method of claim 1 or 2, comprising an aliphatic carboxylic acid.

4. The method of claim 3, wherein the monomer comprises perfluoro-3-butenoic acid.

5. The method of claim 3, wherein the monomer comprises methyl perfluoro-3-butenoate.

6. The method of claim 3, wherein the monomer comprises ethyl perfluoro-3-butenoate.

7. A method according to any one of claims 1 to 6, wherein the fluoropolymer selectively permeable membrane is derived from a copolymer of tetrafluoroethylene and perfluorovinylsulfonyl fluoride.

8. Claims 1 to 7 in which the high-energy radiation is selected from gamma-rays, X-rays, and electron beams.
The method described in any of the preceding sections.

9. The method according to any of claims 1 to 8, wherein the weight increase of the fluoropolymer membrane after graft copolymerization is less than 12%.

10. The method of claim 9, wherein said weight increase is in the range of 3 to 5%.

Description The present invention relates to membranes comprising cation exchange resins and their use in electrolytic cells, in particular chlorine and
The present invention relates to their use as permselective membranes in alkaline electrolysers and to improving the selectivity of such resins and membranes.

Chloro-alkali electrolyzers are used to produce salts and alkali metal hydroxides by electrolysis of alkali metal chloride solutions.

Either mercury tanks or diaphragm tanks are commonly used for this purpose. Although mercury baths have the advantage of producing concentrated alkali metal hydroxides that are substantially free of alkali metal chlorides, problems arise with the disposal of mercury-containing effluents. On the other hand, diaphragm vessels, in which the anode and cathode are separated by a porous diaphragm that passes both cations and anions as well as the alkali metal chloride electrolyte, avoid the aforementioned effluent problems, but (1) , (2) and (3): (1) a relatively weak alkali metal hydroxide solution is obtained which must be evaporated to increase the concentration; (2) the product gas , i.e. the possibility of mixing hydrogen and chlorine, and (3) the resulting alkali metal hydroxide solution comes into contact with a high concentration of alkali metal chloride and the solution must be purified to remove this alkali metal chloride. Things that must be done.

Attempts have been made to overcome the drawbacks of both mercury tanks and diaphragm tanks by using tanks in which the anode and cathode are separated by a cation permselective membrane. Such membranes are permselective and allow only positively charged ions to pass through and prevent the passage of electrolytes and negatively charged ions.

Cation-selective membranes proposed for use in chlor-alkali electrolyte cells include, for example, those obtained from fluoropolymers containing cation exchange groups, such as sulfonate, carboxylate or phosphonate groups and derivatives thereof.

Fluoropolymer, resistant to long-term bath conditions
Preferred are, for example, perfluorosulfonic acid-type membranes, manufactured and sold by EI Dupont under the registered trademark NAFION, which contain perfluoroolefins, such as tetrafluoroethylene, and sulfonate. It is a hydrolyzed copolymer of perfluorovinyl ethers or derivatives thereof containing acid groups. Such membranes are described, for example, in U.S. Pat. No. 2,636,851;
No. 3496077; No. 3560568; No. 2967807; No.
No. 3282875 and British Patent No. 1184321.

Other fluoropolymers used in chlor-alkali electrolysers are manufactured and sold by Asahi Glass Company under the trademark FLEMION and are covered by British Patents Nos. 1516048 and 1522877.
No. 1518387 and No. 1531068.

Such membranes have many desirable properties, such as good long-term chemical stability, which makes them attractive for use in the harsh chemical environments of chlor-alkali baths, although current effects are desirable. It is not very high, especially when sodium hydroxide is produced in high concentrations. As the sodium hydroxide concentration in the catholyte increases, the tendency for backward migration of hydroxide ions into the anolyte increases. This is caused by a drop in current efficiency within the tank. This increases the amount of oxygen impurities in the chlorine, and the build-up of chlorate and hypochlorite contaminants in the brine becomes polysalt, and this contaminant is necessary to maintain acceptable tank operation. Must be removed and disposed of. A current efficiency of at least 90% is highly desirable.

The degree of backward migration of hydroxyl ions is related to the number and nature of active sites in the membrane. However, there is a "trade-off" between the number and nature of active sites, membrane potential drop, and membrane thickness required to provide mechanical strength. That is, the film is thick enough to be used as a membrane with active sites in number and nature such that the film is significantly impermeable to reverse migration of hydroxide ions in a chlor-alkali electrolyte bath. if you did this,
The potential drop is unacceptably high.

For this reason, a multilayer membrane having a layer having high hydroxyl ion impermeability on the catholyte side was developed. Other layers that provide physical strength have high permeability to cations.

Such membranes have been made by lamination methods or chemical modification of active sites or impregnation methods.

Laminated films are disclosed, for example, in US Pat. No. 3,909,378 and US Pat. No. 4,176,21. Laminated membranes have the potential disadvantage of delamination during use due to physical forces resulting from differential swelling of the two-layer composite membrane causing blistering and distortion.

Australian Patent No. 481904 discloses chemically modifying the catholyte surface layer of a membrane having sulfuric acid active sites by treatment with ethylenediamine. This converts sulfonic acid groups to sulfonamide groups for depths up to 75 microns. Another method of chemically modifying sulfonic acid active groups is disclosed in JP-A-53-116287, which discloses converting sulfonic acid groups into carboxylic acid groups. Such methods are complex and difficult to obtain a surface layer of desired thickness.
Furthermore, the chemical stability of membranes under chlor-alkali bath conditions varies adversely, especially for sulfonamides.

The impregnation method is exemplified in JP-A-54-38286.
The catholyte surface of the membrane is coated with monomer, which then polymerizes to form a barrier to the passage of hydroxyl ions. The main drawback of this method lies in the susceptibility of the blocking polymer to leaching by the liquid in the bath. This is because it is only physically associated with the base polymer and not chemically bonded.

Here, we demonstrate how the resistance of fluoropolymer permselective membranes to reverse migration of hydroxy ions is significantly increased by membrane modification, giving a higher fixed charge concentration than that of unmodified membranes. I found out. The modification is a radiation polymerized graft polymerization of a fluorinated unsaturated carboxylic acid monomer onto a fluoropolymer membrane.

"Graft copolymerization" is a term used in "Organic Chemistry of Synthetic Polymers" by RW Lenz.
of Synthetic High Polymers” (Interscience, 1967), p. 251.

Thus, the present invention provides a method for treating a fluoropolymer permselective membrane to improve the membrane's resistance to reverse migration of hydroxide ions when used in a chlor-alkali electrolyzer; The method includes irradiating the membrane with high-energy radiation to generate free radical sites within the membrane;
The method then comprises treating the membrane with a monomeric material comprising a fluorinated unsaturated carboxylic acid or a derivative thereof such that the monomeric material graft copolymerizes with the fluoropolymer to form a copolymerized component within the membrane.

The modified fluoropolymer membranes obtained by the method of the invention have many advantages over modified membranes according to the prior art. In contrast to the sulfuric acid membrane mentioned above, instead of the loss of the number of active sites, there is an increase due to the carboxylic acid groups of the monomers. Thus, there is no need to critically control the depth or degree of modification of the membrane, and the method of the present invention is advantageously applied to substantially the entire depth of the fluoropolymer membrane.

Furthermore, because the method of the present invention leads to chemical bonding of monomers to the fluoropolymer membrane, the resulting modified membrane is superior to prior art modifications that are merely physically associated with the base membrane. It is not susceptible to leaching problems with membranes.

The fluoropolymers to which the method of the invention is advantageously applied are exemplified by what has been indicated above.

The monomeric material that can be radiation-grafted onto the base fluoropolymer membrane by the method of the invention to obtain a modified membrane comprises at least one aliphatic fluorinated unsaturated carboxylic acid or derivative thereof. Suitable carboxylic acids and derivatives have the following general formula: _CF2 =CF-X-COOR {where X is ( _CF2 ) _a or (O- _CH2 - _CH2 -O)
_b - (CF ₂ ) _c (wherein a is 0, 1 or more, b is 0, 1 or more, and c is 0, 1 or more), and R is hydrogen or a lower alkyl group containing up to 6 carbon atoms, preferably up to 3 carbon atoms) and derivatives thereof.

Preferably neither "a" or "c" exceeds 6 and "b" does not exceed 4.

Preferred monomers for use in the present invention include pentafluoro-3-butenoic acid, methylpentafluoro-3-butenoate and ethylpentafluoro-3-butenoate.

Graft copolymerization reactions require high initiation energy. The radiation used in the method of the present invention is 100
It must have a wavelength of angstroms or less and an energy value of 100eV or more. The radiation preferably used to initiate the graft copolymerization is γ-rays, X-rays or electron beams; γ-rays, X-rays or electron beams;
- lines are the preferred form.

There are several preferred techniques for radiation grafting that can be used in the method of the present invention. For example, the membrane is immersed in a liquid phase containing the monomers to be graft copolymerized with the monomers and then subjected to continuous or intermittent irradiation, preferably in the absence of oxygen. An alternative method, which is somewhat less preferred, is to pre-irradiate the membrane before contacting it with the liquid containing the monomeric material.

The monomeric material may be liquid itself or in a suitable solvent, preferably methanol, toluene, 1,
1,2-Trichlorotrifluoroethane ( _CFCl2
-CF ₂ Cl) or may be dissolved in water.

The nature of the reaction vessel used to carry out the process of this invention is not narrowly limited and will be apparent to those skilled in the art of radiation polymerization. One technique that has been found to be particularly effective will be described with reference to FIG. A monomer sample 1 in liquid form is contained in a container 2 having an opening at the top. A sample of membrane material 3 extends across the aperture and is sealed with a sealing ring 4. The monomer liquid contacts the membrane to form a vapor and the monomer material within the vapor phase graft copolymerizes with the membrane under the action of high energy radiation.

Optionally, the method of the invention is used to treat one or both sides of a fluoropolymer membrane, particularly the side exposed to the catholyte in a chlor-alkali bath.
When the method of the invention is applied to one side of the membrane, it is conveniently carried out by the technique described with reference to FIG.

The two pieces of permselective membrane 11 to be treated are mounted together in continuous contact with each other. They are,
It is helically wound onto a spool 12 and mounted within a hollow cylindrical cylinder 13. For clarity, the two pieces are shown separately in FIG. 2, but in fact they are in contact with each other. The spiral turns are separated by spacers 14. The hollow cylinder 13 with the two membranes is filled with a monomeric substance in liquid or vapor form. The entire membrane is irradiated so that graft polymerization occurs on the exposed surface of each membrane. After treatment, the membrane is unwound and separated.

The method is particularly effective when applied to membranes of the "NAFION" type, ie derived from the copolymerization of tetrafluoroethylene and perfluorovinylsulfonyl fluoride. The amount of monomer grafted is determined in part by the desired improvement in the performance of the fluoropolymer membrane. For large and low cases, significant performance improvements are achieved by grafting 3-5% w/w of monomer (measured as membrane weight gain after grafting), and preferably 12% w/w of monomer. Less than w is introduced.

The modified membranes obtained by the method of the invention are more effective in preventing the reverse migration of hydroxide ions from the catholyte compartment of a chlor-alkali tank than the precursor membranes from which they are made. It is. Precursors from which they were made under similar operating conditions in chlor-alkali baths.

Under similar operating conditions in a chlor-alkali tank,
They exhibit high current efficiency.

The reforming membranes of the present invention are useful in other electrochemical systems, such as solid electrolytes in separators and/or batteries, fuel cells, and cells.

The invention is illustrated in a non-limiting manner by the following examples, in which all parts and percentages are by weight unless otherwise stated.

Example 1 A film of "NAFION" 390 is mounted in an apparatus as shown in FIG _.
CF CF ₂ COOH). The entire body was heated at 25°C in an oxygen-free atmosphere.
It was exposed to γ-irradiation from a Co ⁶¹ source at a dose rate of 6 krad/h. The absorbed dose was 2.5 Mrad. The weight of the film increased by 4.1%. The treated film is 30% w/w before being installed in the electrolytic test chamber.
w of sodium hydroxide solution, the tank was heated to 25 w.
% w/w sodium hydroxide solution was produced in the catholyte chamber and chlorine was released at the anode.

The current efficiency (ie, the weight of sodium hydroxide produced expressed as a percentage of the theoretical yield of sodium hydroxide equivalent to the applied current) under steady operating conditions was 93%. The current efficiency of electrolysis with untreated NAFION 390 membrane under similar conditions was 80%.

This demonstrates the utility of the method of the invention.

Example 2 The conditions of Example 1 were repeated. However, the γ-irradiation dose was increased to 15 krad/hr, but the irradiation time was decreased so that the absorbed dose was 1.5 Mrad.

The weight increase of the film was 4.4%.

The current efficiency measured under electrolysis operating conditions similar to Example 1 was 92%.

Example 3 The conditions for treatment of membranes described in Example 2 were repeated and the treated membranes were tested in a test electrolysis tank operating at high sodium hydroxide concentrations (30-34% w/w) in the catholyte. . Again the current efficiency was 92%.

Examples 4-7 Samples of NAFION membranes were tested in a manner similar to that described in Example 1. However, the monomer used was methylpentafluoro-3-butenoate ( _CH2 = _{CFCF2COOCH3 )} .

The current efficiencies obtained in treatment with membranes derived from two grades of "NAFION" were recorded for various sodium hydroxide catholyte solutions in test electrolyzers.

Regarding the unprocessed film of "NAFION" 110,
The current efficiency of the electrolyzer operating at a 20% w/w sodium hydroxide concentration was 58%.

[Table] Example 8 The procedure of Example 1 was repeated, but the film used was "NAFION" 117 and the irradiation dose rate was 3.5 krad/hour to obtain a total absorbed dose of 359 krdrd.

The weight of the film increases by 4.7% and 30%
The current efficiency was 60% for the electrolyzer operating at a w/w sodium hydroxide concentration. Untreated "NAFION" 117 film meets these conditions by 44%
The current efficiency was .

Example 9 The procedure of Example 8 was repeated except that methyl pentafluoro-3-butenoate was used in place of pentafluoro-3-butenoic acid. The weight of the film increased by 3.5% and the current efficiency under the same electrolytic operating conditions was 55%.

Example 10 The procedure of Example 8 was repeated except that ethyl pentafluoro-3-butenoate was used in place of pentafluoro-3-butenoic acid.